A known transmission interference filter employs a stack of alternating silicon and silicon dioxide (SiO2) layers. Such devices are known for use in the short wave and mid wave infrared down to about 1100 nm, as both silicon and SiO2 are transparent in this range. The lower wavelength threshold (corresponding to the upper photon energy threshold) is controlled by the onset of absorption by the silicon, which in its crystalline form has a bandgap of about 1.12 eV. A key advantage of silicon in these devices is its high refractive index. The spectral profile of an optical interference filter is, among other things, dependent on the angle of illumination. As the angles increase the filters shift to shorter wavelength. This angular shift is dependent on the materials used and the distribution of those materials. Higher refractive index results in less angle shift. For narrow band filters the amount of angle shift limits the useful bandwidth of the filter when used in optical systems. In systems with large angular acceptance angles a filter constructed such as to yield low angular shift can have a narrower passband and hence greater noise rejection than one constructed of materials with lower refractive index.
To extend device operation into the near infrared, it is further known to hydrogenate the silicon, so as to employ alternating layers of hydrogenated amorphous silicon (a-Si:H) and SiO2. By hydrogenating the silicon, the material loss and refractive index are reduced. By this approach, very high performance interference filters operating in the 800-1000 nm range are achievable.
Some improvements are disclosed herein.